The present invention pertains to microlithography performed using a charged particle beam such as an electron beam. More specifically, the invention pertains to reticles as used for such microlithography.
In recent years the progressive miniaturization of semiconductor integrated circuit elements has led to current efforts to develop a practical projection-exposure system (microlithography system) that utilizes either an X-ray or a charged particle beam (e.g., electron beam or ion beam) as an energy beam. The reason is because optical microlithography (i.e., microlithography using light, especially ultraviolet light) has resolution limits that make it extremely difficult to impossible to resolve circuit elements as small as currently desired. Electron-beam microlithography offers prospects of achieving the currently desired pattern-element resolution (0.1 xcexcm or less) because an electron beam can be focused to a diameter of a few nanometers.
Conventional electron-beam exposure systems xe2x80x9cwritexe2x80x9d a pattern onto a wafer or other substrate one line at a time. Hence, the finer the pattern the more focused the electron beam must be. Also, the finer the pattern the longer the time required to draw the pattern. In fact, the time required to draw a pattern line-by-line is so long that the electron-beam-drawing technique cannot be used to expose wafers for mass production.
In view of the low xe2x80x9cthoughputxe2x80x9d (number of wafers processed per unit time) and hence low cost-efficiency of electron-beam drawing technology, considerable effort currently is being expended to develop a practical electron-beam projection-microlithography system in which an image of a pattern, defined by a reticle, is projected (rather than written) from a pattern-defining reticle onto the wafer. The projected image typically is xe2x80x9creducedxe2x80x9d or xe2x80x9cdemagnifiedxe2x80x9d, by which is meant that the image is smaller (usually by an integer factor) than the corresponding pattern on the reticle. The image is projected onto the wafer using a projection lens.
To perform projection microlithography of a circuit pattern, a transfer mask (xe2x80x9creticlexe2x80x9d) is required upon which the circuit pattern is formed (i.e., the reticle xe2x80x9cdefinesxe2x80x9d the pattern). A first representative conventional reticle is a scattering-membrane reticle 31 as shown in FIG. 3(a). In the scattering-membrane reticle 31, the pattern is defined by a corresponding arrangement of xe2x80x9cscattering bodiesxe2x80x9d 34 formed on a membrane 32. The scattering bodies 34 are respective portions of a layer of a material (e.g., heavy metal) that scatters incident electrons. The membrane 34 is relatively transmissive to the electron beam irradiating the upstream-facing surface of the reticle, whereas the scattering bodies 34 tend to scatter electrons incident on the reticle. A second representative conventional reticle is a scattering-stencil reticle 41 as shown in FIG. 3(b). In the scattering-stencil reticle 41, the pattern is defined by a corresponding arrangement of through-holes (xe2x80x9cvoidsxe2x80x9d) 44 defined in a membrane 42. The membrane 42 is typically thicker than the membrane 34 in the scattering-membrane reticle 31 so as to exhibit substantial scattering of electrons in a beam incident on the upstream-facing surface of the reticle 41.
Due to the current impossibility of simultaneously exposing an entire reticle at one instant using a charged particle beam, conventional CPB-microlithography reticles typically are xe2x80x9cdividedxe2x80x9d or xe2x80x9csegmentedxe2x80x9d into multiple small regions (xe2x80x9csubfieldsxe2x80x9d or xe2x80x9cexposure unitsxe2x80x9d). In FIG. 3(c), each subfield on a scattering-membrane reticle 34 is denoted by the reference numeral 32a, and each subfield on a scattering-stencil reticle 41 is denoted by the reference numeral 42a. Each subfield 32a, 42a defines a respective portion of the overall pattern defined by the respective membrane 32, 42. A representative subfield 32a is shown in FIG. 3(a). The subfields are separated from one another by boundary regions 35 in which no pattern features are defined. Extending from each boundary region 35 is a support strut (item 33 in FIG. 3(a)) that provides physical support for the membrane 32. Reference is also made to FIG. 4 showing support struts 43 on a scattering-stencil reticle 41. The support struts 33, 43 form a criss-cross network on the respective reticle, wherein the subfields 32a, 42a are located between the support struts 33, 43.
In a conventional scattering-stencil reticle the membrane 42 typically is a silicon membrane about 2xcexcm thick. As noted above, the membrane 42 defines the through-holes that are transmissive to the incident electron beam.
Conventionally, the area of the reticle that can be exposed at any instant by the incident electron beam is about 1 mm square. Hence, each subfield must define a respective portion of the overall pattern to be transferred to a particular region (xe2x80x9cdiexe2x80x9d) on the wafer, wherein a die corresponds to the area occupied by a xe2x80x9cchipxe2x80x9d as formed on the wafer.
As indicated in FIG. 3(c), pattern transfer is conventionally performed by illuminating the subfields 32a, 42a with the incident charged particle beam. The subfields 32a, 42a are typically exposed sequentially in a stepwise manner. As each subfield is illuminated for exposure, the corresponding portion of the pattern is demagnified and transferred to the xe2x80x9csensitive substratexe2x80x9d (wafer) 37 by a projection-optical system (not shown). The images of the subfields 32a, 42a are formed on the wafer 37 in respective locations in which the images are properly xe2x80x9cstitched togetherxe2x80x9d (contiguously arranged) with no intervening boundary regions.
In a conventional segmented reticle as described above, each support strut typically has a width of approximately 180 xcexcm. The cumulative effect of having to dedicate a substantial portion of the reticle to non-pattern-defining struts is an excessively large reticle. Furthermore, during manufacture of such a reticle in which the struts are formed by etching, it is difficult to satisfactorily control the width of such support struts.
Moreover, the resulting large reticle must be mounted on and conveyed by a correspondingly large reticle stage. A suitably large reticle stage has a substantial mass that requires correspondingly large and robust stage-actuating mechanisms for moving the reticle as required for exposure.
One conventional approach for reducing the size of a segmented reticle is to arrange groups of subfields into rows, wherein each row of subfields is separated from other rows by support struts. Thus, each row contains multiple subfields situated side-by-side. (Such a reticle is regarded as having a xe2x80x9cslotxe2x80x9d configuration.) In order to scan a row of subfields, an electron beam is deflected in a lateral sweeping manner.
The positional accuracy of such scanning desirably is 0.5 xcexcm or less. Unfortunately, maintaining such positional accuracy is not possible from the perspective of achieving adequate digital-to-analog (DAC) conversion of energizing signals routed to the respective deflectors in the electron-optical system. Also, the variation in positional accuracy of the electron beam is not uniform in conventional practice, resulting in double-exposed portions or non-exposed portions of the pattern as projected onto the wafer. These problems are manifest as xe2x80x9cstitchingxe2x80x9d errors of the pattern as projected onto the wafer. Also, the continuously scanning electron beam must be rigorously controlled during exposure so as to achieve accurate stitching and to compensate for variations in pattern-element density and shape configurations from one subfield to the next. That is, the electron-optical system must be controlled in a manner allowing continuous high-speed processing. However, achieving such control is conventionally extremely problematic.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide, inter alia, reticles for charged-particle-beam (CPB) microlithography that achieve the desired level of stitching accuracy while minimizing the overall size of the reticle.
As a result of thorough research, the inventor has discovered the optimal width of a non-patterned region located between subfields in a row of subfields in a slot reticle. A representative embodiment of such a reticle comprises a reticle membrane that defines a pattern to be projection-transferred from the reticle to a sensitive substrate. The reticle also comprises support struts configured to divide the membrane into multiple rectangular regions each defining a respective portion of the pattern. Each rectangular region comprises a longitudinal array of respective subfields (usually square in shape) each defining a respective portion of the pattern. Each subfield in a rectangular region is separated from adjacent subfields in the rectangular region by intervening non-patterned regions each having a width of 1 xcexcm to 50 xcexcm.
The subject reticle can be a scattering-membrane reticle or a scattering-stencil reticle. The non-patterned regions desirably are defined in a layer (e.g, 50 nm thick) of heavy metal such as gold, platinum, or tungsten.
According to another aspect of the invention, methods are provided for performing CPB microlithography. In a representative embodiment of such a method, a reticle is provided such as summarized above. The reticle is illuminated and projected as summarized above onto a sensitive substrate.
The foregoing and additional features and advantages of the invention will be more readily understood from the following detailed description, which proceeds with reference to the accompanying drawings.